The present invention relates to a method for removing nitrogen and phosphorous in wastewater, more specifically, to a method for removing nitrogen and phosphorous simultaneously from wastewater by employing fermentation broth obtained by anaerobic fermentation of organic waste materials.
The presence of nitrogen and phosphorous compounds in wastewater is one of the primary causes of eutrophication of lakes and rivers. However, since most wastewater treatment plants are concentrated on removing organic compounds, it is urgently necessary to develop new and efficient treatment systems or facilities for the removal of nitrogen and phosphorous compounds.
In general, removal of nitrogen compounds in the wastewater can be achieved in a two-step biological reaction: The first step is nitrification by the autotrophs which converts ammonia nitrogen into nitrite and nitrate under aerobic condition; and, the second step is denitrification by the heterotrophs which reduce oxidized nitrogen compounds into nitrogen gas under anoxic condition(see: Castens, D. J. and Rozich, A. F., Biotechnol. Bioeng., 18:461-465(1985)), which requires organic carbon compounds as electron donors. However, since the wastewater does not contain sufficient organic carbon compounds, addition of organic carbon compounds to the wastewater treatment system is indispensable for the complete removal of nitrogen. In this regard, several organic carbon compounds such as methanol, glucose and acetate, have been tested as a carbon source, and acetate is known to be the most effective compound among them (see: Tom N.F.Y., Wong Y. S. and Leung G., Wat. Res., 26(9):1229-1236(1992)). Although methanol is more frequently used as the electron donor due to its inexpensive cost, it is urgent to find a new alternative carbon compound for the methanol, as an effort to reduce operation cost in the wastewater treatment plants.
Under the circumstances, several attempts have been made to use various industrial wastes in the art(see: Skrinde J. R. and Bhagat S. K., J. WPCF, 54(1):370-377(1982); N. Bernet, et al., Appl. Microbiol. Biotechnol., 46:92-97(1996)). However, the industrial wastes have revealed some shortcomings in the senses that: they do not contain proper organic compounds necessary for denitrification; the components in the wastes are not consistent or reliable; and, they tend to contain non-degradable compounds. Furthermore, since the industrial plants, in most cases, are located far away from the wastewater treatment plants, the cost for transportation prevents the actual use of the processes.
On the other hand, removal of phosphorous can be achieved by the aid of microorganisms, which secrete phosphate as they store organic materials under anaerobic condition, but accumulate phosphate as polyphosphates under aerobic condition(see: Levin G. V. and Shapiro J., J. WPCF., 37:800(1965)). By alternating anaerobic/aerobic conditions, phosphorous uptake by the microbes can be accelerated. Since these polyphosphates accumulating microbes require short chain fatty acid as a carbon source, the influent wastewater composition is one of the most important factors for the efficient phosphate removal from wastewater.
Currently known processes for simultaneous removal of nitrogen and phosphorous includes A2/0 process, five-stage Bardenpho process, UCT(University of Cape Town) process, and VIP(Virginia Initiative Plants). These processes are modified processes of conventional activated sludge methods and differ in their arrangement and flow of the processes of aerobic, anaerobic and anoxic conditioned reactors(see: Tchobanoglous G. and Burton F. L., Wastewater Engineering, 3rd ed., McGraw-Hill(1991)). They all utilize organic materials in the influent wastewater, so that without additional carbon source the nitrogen in the wastewaters cannot be removed lower than a certain limit.
Japanese patent publication No. 82-12437 discloses a method for the simultaneous removal of nitrogen and phosphorous by using organic acids obtained from biologically treated sludge. However, since the nitrogen and phosphorous contents in the sludge are as high as 10% and 2.2%, it has limits on the efficient removal of nitrogen and phosphorous from wastewater.
Russian patent No. 1,255,587 also describes a method for removing nitrogen by using organic acid obtained from organic waste, though it has the same problems as the one in Japan.
Korean patent laid-open publication No. 95-212 describes a method for production of organic acids by anaerobic fermentation of solid waste separated from the influent wastewater. The prior art method is, however, proven to be less satisfactory in the senses that: it requires additional reactors to remove ammonia nitrogen; and, it is ineffective in phosphorous removal. Also, Korean patent laid-open publication No. 95-8039 discloses a denitrification process, which employs anaerobic fermentation broth from the mixture of settled solid waste and surplus sludge in the wastewater treatment plant. However, since the fermentation broth contains high levels of nitrogen and phosphorous, it is essentially required to add chemicals such as MgO to remove nitrogen and phosphorous.
Accordingly, there are strong reasons for developing an economical and efficient method for removing nitrogen and phosphorous compounds from wastewater, which can solve the various problems of previous systems.
In accordance with the present invention, the inventors have tried to develop a novel method for removing nitrogen and phosphorous from wastewater in an economical and efficient manner, and found that nitrogen and phosphorous can be removed simultaneously by using anaerobic fermentation broth of organic wastes with low nitrogen and phosphorous contents.
A primary object of the invention is, therefore, to provide a method for removing nitrogen and phosphorous in wastewater.
The other object of the invention is to provide a fermentation broth with low nitrogen and phosphorous contents, which is obtained by anaerobic fermentation of organic solid wastes.